The art of lithographic printing is based upon the immiscibility of oil and water, wherein an oily material or ink is preferentially retained by an imaged area and the water or fountain solution is preferentially retained by the non-imaged areas. When a suitably prepared surface is moistened with water and ink is then applied, the background or non-imaged areas retain the water and repel the ink while the imaged areas accept the ink and repel the water. The ink is then transferred to the surface of a suitable substrate, such as cloth, paper or metal, thereby reproducing the image.
Very common lithographic printing plates include a metal or polymer support having thereon an imaging layer sensitive to visible or UV light. Both positive- and negative-working printing plates can be prepared in this fashion. Upon exposure, and perhaps post-exposure heating, either imaged or non-imaged areas are removed using wet processing chemistries.
“Direct-write” imaging generally uses infrared radiation from a laser source. More particularly, a computer-controlled infrared laser imagewise exposes small regions of a thermally sensitive composition to produce an image area, pixel by pixel.
Such thermally sensitive printing plates are becoming more common. Examples of such plates are described in U.S. Pat. No. 5,372,915 (Haley et al.). They include an imaging layer comprising a mixture of dissolvable polymers and an infrared radiation absorbing compound. While these plates can be imaged using lasers and digital information, they require wet processing using alkaline developer solutions.
It has further been recognized that such direct writing techniques may be utilized in the formation of “processless” printing plates. As used herein, the term “processless” refers to imaging materials that do not require one or more conventional processing steps (for example development) prior to mounting on a printing press. In some publications, such imaging materials are known as “printing plate precursors”.
One method for forming processless printing plates is through ablation of a thermally sensitive layer. Thus, it has been recognized that a lithographic printing plate could be created by ablating an IR absorbing layer. For example, Canadian 1,050,805 (Eames) discloses a dry planographic printing plate comprising an ink receptive substrate, an overlying silicone rubber layer, and an interposed layer comprised of laser energy absorbing particles (such as carbon particles) in a self-oxidizing binder (such as nitrocellulose). When such plates were exposed to focused, near IR radiation using a laser, the absorbing layer converts the infrared energy to heat thus partially loosening, vaporizing, or ablating the absorber layer and the overlying silicone rubber. Similar printing plates are described in Research Disclosure 19201, 1980 as having vacuum-evaporated metal layers to absorb laser radiation in order to facilitate the removal of the silicone rubber overcoat layer.
While the noted abatable printing plates used for digital, processless printing have a number of advantages over the more conventional photosensitive printing plates, there are a number of disadvantages with their use. The process of ablation creates debris and vaporized materials that must consequently be collected. Moreover, the laser power required for ablation can be considerably high, and the components of such printing plates may be expensive, difficult to coat, or unacceptable for resulting printing quality.
Thermal or laser mass transfer is another method of preparing processless lithographic printing plates, as described for example, in U.S. Pat. No. 5,460,389 (Peterson) wherein a hydrophobic image is transferred from a donor sheet to a microporous hydrophilic crosslinked silicated surface of a received sheet.
Thermally switchable polymers have been described for use as imaging materials in processless printing plates. By “switchable” is meant that the polymer is rendered from hydrophobic to relatively more hydrophilic or, conversely from hydrophilic to relatively more hydrophobic, upon exposure to heat. EP 0 652,483 (Ellis et al.) describes imaging materials of this type.
U.S. Pat. No. 6,190,830 (Leon et al.), U.S. Pat. No. 6,190,831 (Leon et al.), and U.S. Pat. No. 5,985,514 (Zheng et al.) are directed to processless direct write imaging members that include an imaging layer containing heat sensitive ionomers. The polymer coatings are sensitized to infrared radiation by the incorporation of an infrared absorbing material such as an organic dye or a fine dispersion of carbon black. Upon exposure to a high intensity infrared laser, light absorbed by the organic dye or carbon black is converted to heat, thereby promoting a physical change in the ionomer (usually a change in hydrophilicity or hydrophobicity). The imaged materials can be used, for example, on conventional printing presses to provide negative images. Such printing plates have utility in the evolving “direct write” printing market.
Heat-sensitive compositions and imaging members that include heat-decomposable microcapsules are described in U.S. Pat. No. 5,569,573 (Takahashi et al.). U.S. Pat. No. 4,970,247 (Hoppe et al.) describes lacquers that are formulated from particles containing cellulose esters and polymerized monomers that are dispersed in a continuous phase.
A printing material containing an imaging layer that contains nitrocellulose particles that are encapsulated with polystyrene dispersed in a binder is described in U.S. Pat. No. 5,324,617 (Majima et al.). Printing plates containing thermoplastic particles are described, for example, in U.S. Pat. No. 6,106,996 (Van Damme et al.) and EP 0 514 145A1 (Matthews et al.). These particles coalesce upon application of thermal energy.
The graphic arts industry is constantly seeking alternative means for providing direct-write lithographic imaging members that can be readily imaged in “direct-write” printing using digital information without ablation and conventional wet post-imaging processing steps.